Shanise Hamilton had a tough childhood. Born into a single-parent home, Hamilton never knew her biological father.

"My biological mom dealt with substance abuse for years and it took over to the point where we were left at home days by ourselves, me and my older brother … the days that we did eat a lot of times it was because he went out to a neighbor's house and said, 'We need food,'" Hamilton said.

When she was four years old, she ended up in foster care.

"Growing up in the foster care system, I constantly heard I was not going to amount to anything," she said.

She proved them wrong.

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Andrew Donnellon was born with just half a heart. Doctors told his mother, Sherri, that the odds of her son's surviving to age 5 were just 50 percent.

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Lorie Kiss lost the love of her life to an aggressive form of brain cancer. Before her husband, JJ, died, she promised him she would find a way to have his child.

Today, she's eight months pregnant with JJ's child.

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What do Shanise Hamilton, Andrew Donnellon and Lorie Kiss all have in common? They overcame tremendous obstacles and became an inspiration to others, including to "Good Morning America" co-anchor Robin Roberts. Roberts chose their stories from among the thousands that were submitted when "GMA" asked people to share their experiences of triumphing over tremendous hardships or achieving a cherished dream.

So just how did Hamilton, Donnellon and Kiss do it?

Although doctors had made their dire prognosis about Donnellon's future, he had other plans.

As a child, he had boundless energy and feared nothing. He played soccer and baseball, and thrived.

When he got to high school and said he wanted to try to be a kicker on the high school football team, his parents weren't so sure about that. But Andrew's doctor, and then his coach, both gave their approval.

"I had never kicked a football before in my life before I went to practice," Andrew Donnellon, now 18, said. "I just thought I could do it, and it turns out I was kind of decent at it."

A high school teammate who learned about Donnellon's condition came up with a clever nickname for him: Tin Man.

A reference to the character in "The Wizard of Oz" who longed to have a heart, the nickname stuck. Donnellon turned out to be such a good player that he was recruited as a kicker for Bluffton University.

"If you … told me five years ago that I would grow up to be a college football player, I would have just laughed," the Cincinnati native said.

Added his mother: "He may have only been born with half a heart, but to me he's got the biggest heart of anyone I've known. "

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Lorie Kiss, of Playa Vista, Calif., was determined to keep the promise she made to her husband, JJ Allen, as he lay dying.

After he had successfully battled a benign brain tumor, three years later he was diagnosed with one of the most aggressive types of brain cancer. Doctors said he was terminally ill and gave him a year to live.

"They recommended in the slight chance that he survived and he wanted to have children in the future, that we would have to freeze his sperm before starting chemotherapy," she said.

"While he was in that bed, we both made promises to each other … I promised him, I was like, 'I don't know how it's going to happen but I'm having your babies at some point in my life," Kiss recalled.

In December 2003, JJ died at the age of 28. Seven years later, Kiss met Kathryn Feller, and they got married.

Kiss's wife was more than willing to help her keep her promise to JJ.

"I'm just about eight months pregnant and we're expecting our little boy," Kiss said. "Ten years later after [JJ's] passing, this is my ultimate promise to him and it's finally coming true."

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Shanise Hamilton was adopted when she was 7, but she spent years going from one makeshift home to the other.

At one point she was homeless, and lived in a car when she could no longer afford a motel.

"There are times that I've been alone and I've just cried for hours praying to God, 'Just please just hug me, just show me that you're here,'" she said.

Now 25 years old, Hamilton is working as a producer and on-air personality at a radio station in Gainesville, Fla.

She has a very simple message for people who are struggling.

"There are going to be days when all you have is yourself and you have to just believe that it's going to get better," she said. "Just keep fighting, keep moving and keep praying."